This invention relates to a fuel metering valve forming part of an internal combustion engine feed device. Devices of this type are arranged to deliver a predetermined quantity of fuel, which mixes with the air in the internal combustion engine feed manifold, and comprise substantially mixture forming means to which, by way of suitable ducts, air is fed together with predetermined quantities of fuel, this latter by means of the said metering valve.
This latter comprises substantially a casing in which a path is defined for the fuel which is fed to the valve by the action of a delivery pump, and a valving member which is mobile axially within the casing between a first position in which it makes contact with a suitable seat to close said path, and a second position in which said path is open. The valving member is operated by an electromagnet disposed in the casing, and is rigid with an armature which is attracted by the electromagnet core. In this casing there is disposed a ring to form a stop for the valving member when in its second aforesaid position. This ring is normally interposed between a casing shoulder and the front surface of a nosepiece which is rigid with the casing and houses the valving member.
This latter is retained in the first said position (closed position) by the action of a spring, whereas when the electromagnet is energised the valving member moves axially to open the fuel path until it stops against the said ring. In this second position, the residual space (residual air gap) remaining between the armature rigid with the valving member and the electromagnet core is usually very small. When the electromagnet energisation ceases, the valving member returns to its first position to close the fuel path.
Although valves of the described type operate satisfactorily, they give rise to certain difficulties during their assembly. To understand how such difficulties arise, it should be noted that the stroke through which the valving member travels in moving from the first to the second of the aforesaid positions and the residual space (residual air gap) between the armature and core in this second position must be adjusted in an extremely precise manner with a tolerance of only a few thousandths of a millimeter. Only if these conditions are satisfied can the fuel be precisely metered and the electromagnetic action of the electromagnet be applied correctly.
In valves of the indicated type, the stroke and air gap are adjusted in the following manner. The valve is initially assembled, and after assembly the stroke of the valving member and the air gap are measured with suitable tools. If these are incorrect, the valve is dismantled and the valving member stop ring is replaced, the thickness of this latter determining the air gap between the armature and core. To adjust the stroke of the valving member, a predetermined quantity of material is removed from the front surface of the nosepiece, facing said ring. This thus reduces the distance between said surface and the seat within the nosepiece against which the valving member rests when in its closed position, to thus vary the stroke of this latter.
When carried out in the aforesaid manner, the stroke and air gap adjustments not only require successive valve assembly and dismantling, but also require chip-forming machining on certain parts of the valve, and are therefore lengthy and costly, and require special care and ability.